This application is based on French Patent Application No. 97/08176, filed on Jun. 23, 1997, which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to the field of printing using photosensitive inks, i.e. inks which can be dried or polymerised by light radiation, notably ultraviolet radiation.
2. Related Background
Printing on supports such as plastics materials which do not absorb traditional inks based on water, alcohol or oil has been made possible by developing solvent-based inks adapted to these materials and concurrently polymeric inks capable of solidifying and adhering to the material.
A prohibitive drawback of solvent-based inks is the harmfulness of the solvents used, of the acetone type. Printing with such inks requires complex devices collecting the solvents given off and major precautions in use.
Polymeric inks do not have these drawbacks in use and lend themselves particularly well to printing dot by dot, notably by inkjet.
In the liquid phase, these inks have a fluidity which makes it possible to mechanically deposit, notably in an offset process, ink drops of very fine size, or to spray drops dot by dot onto a support.
The definitive fixing of polymeric inks is effected during a so-called ink cross-linking step which follows the deposition of the ink drops.
Cross-linking consists in polymerising or crystallising the ink, the polymers making up the ink being bonded together in order to form longer polymer chains and to be fixed to the support. A cross-linking step therefore enables the ink to be solidified and fixed to the support.
The supports consisting of plastics material, such as polyvinyl chloride (PVC), polyethylene (PE), polyethyltetraethylene (PET), polycarbonates (PCs), acrylonitrile-butadiene-styrene (ABS) and other organic polymers are quite naturally suited to printing by polymer ink, the polymers in the ink and the polymers in the support being firmly fixed together during the cross-linking.
Cross-linking is obtained by exposure of the ink support to ultraviolet radiation. Ink which can be cross-linked to ultraviolet radiation, abbreviated to UV ink, will therefore be spoken of hereinafter. The energy of the ultraviolet photons allows polymerisation of the polymer chains with each other. However, the support must be exposed to a sufficient ultraviolet radiation power and for a sufficient length of time for the ink to be well fixed to the support and to harden completely.
FIGS. 1 and 2 diagrammatically show known techniques of printing using cross-linkable UV ink. FIG. 1 shows diagrammatically a multicolour offset printing of a support. The support 10 advances between a drive cylinder 15 and contact printing rollers 11, 12, 13 and 14. Each roller 11 or 12 or 13 or 14 contains a screen of the image to be printed. The hollows in the screens on each roller are inked with a black ink or coloured ink, notably cyan, magenta or yellow. Several screens of colour are thus deposited on the support in order to constitute a final multicolour image. The inking step is followed by a step of cross-linking by continuous exposure 19 of the support 10 under an ultraviolet lamp 18. Naturally the offset printing can be monochrome by providing a single black or colour inking roller.
FIG. 2 shows diagrammatically a method of multicolour printing by inkjet. Several reservoirs 21, 22, 23 and 24 containing the black polymer ink and those of different colours feed at least one nozzle ejecting drops of ink, each reservoir preferably having its own line of ejection nozzles, the printing line being transverse to the direction of movement of the support. The ink drops are deposited dot by dot on the support, a device for moving the support and for the computer programming of the image to be printed controlling the ejection of the drops through each nozzle in the line with if necessary control of the drop volume ejected. The computer system defines the spatial rotation of the points to be inked and controls the ejection or non-ejection of the drops according to this location. The inking of the support 20 is followed by a cross-linking step, still with continuous exposure, the support moving forward under an ultraviolet lamp. FIG. 2 illustrates an alternative printing in which each inking step is followed by a cross-linking step in order to dry each ink before a subsequent inking of a different colour. The printing device of FIG. 2 therefore has in this example four ultraviolet lamps 25, 26, 27 and 28 for drying each ink individually.
In order to increase the printing rates, it has been proposed to increase the power of the ultraviolet lamps, thus reducing the support exposure time, the support still receiving sufficient energy to dry and fix the ink.
However, ultraviolet lamps release a great deal of heat. Printing devices with polymerisable ink must therefore include an expensive and bulky cooling system. The adoption of so-called cold UV lamps, designed to emit less infrared radiation and therefore less heat, do not dispense with the need to have cooling when high printing rates are required.
A drawback of the known printing devices with ink which can be cross-linked by ultraviolet radiation is therefore the high release of heat during the cross-linking steps.
Another drawback is the premature aging of the supports and their yellowing under the effect of the cross-linking ultraviolet radiation.
One aim of the invention is to provide an ink cross-linking method allowing printing at high rate, without the aforementioned drawbacks.
A particular aim of the invention is to prevent the yellowing of the support in order to afford durable printing of high quality.
Succinctly, these aims are achieved, according to the invention, by providing for the cross-linking to be carried out by an ultraviolet laser beam concentrated on the ink drops deposited on the surface of the support, the white surfaces of the support not being swept by the laser beam.
The invention is implemented by providing a method for the cross-linking of photosensitive ink including a step of inking points on a support and a particular step consisting in applying an ultraviolet beam concentrated on the ink dots, to the exclusion of the non-inked surfaces of the support.
The inking step preferably consists in depositing, dot by dot on a printing support, drops of polymerisable ink, the ink being polymerisable by ultraviolet radiation.
The invention is preferably implemented by the application of an ultraviolet laser beam.
A first embodiment of the invention provides for the application of the laser beam to be effected by dot-by-dot sweeping of the support.
A second embodiment of the invention provides for the application of the ultraviolet beam to be effected by means of an optical fibre or an array of optical fibres.
According to a preferred characteristic of the invention, provision is made for interrupting the ultraviolet beam when it is directed towards the non-inked surfaces of the support, one embodiment of the invention being able to include continuous sweeping of the support.
According to an alternative characteristic, provision is made for modulating with respect to power the ultraviolet beam concentrated on the inked dots.
The invention applies particularly to printing and cross-linking of ink on a support made of plastics material.
Advantageously, the method of cross-linking ink according to the invention applies particularly to a method of printing dot by dot by inkjet and/or a multicolour printing process.